New Zealand flood design now requires climate change uplift, but how much uplift, and which RCP scenario to apply, depends on your council, your catchment, and your intended design life. HIRDS (High Intensity Rainfall Design System) provides the rainfall frequency data, but converting that to a flood peak requires knowing which return period to combine with which RCP pathway. Getting this wrong means either an under-designed structure or an over-engineered one, and councils are increasingly scrutinising the methodology.
What HIRDS provides
HIRDS is NIWA's online tool for estimating rainfall intensity at any location in New Zealand for a range of return periods (2-year to 100-year and beyond) and storm durations (10 minutes to 72 hours). It draws on the national rainfall gauge network and provides depth-duration-frequency (DDF) tables that form the basis of most flood design in the country.
Critically, HIRDS v4 includes climate change uplift factors for four RCP scenarios (2.6, 4.5, 6.0, and 8.5) at two future time horizons (2031-2050 and 2081-2100). These factors increase the historical rainfall intensities by a percentage that reflects projected changes in atmospheric moisture under each emissions pathway.
What the RCP scenarios mean in practice
RCP stands for Representative Concentration Pathway. The number after "RCP" represents the radiative forcing in watts per square metre by the year 2100. In practical terms for flood design:
- RCP 2.6: Strong emissions reduction. Rainfall uplift is modest, typically 5 to 10% by 2081-2100 for short-duration storms in Hawke's Bay.
- RCP 4.5: Moderate emissions reduction. Uplift factors are in the range of 8 to 15%.
- RCP 6.0: Moderate-to-high emissions. Uplift factors of 10 to 20%, depending on duration and location.
- RCP 8.5: High emissions, no significant mitigation. Uplift factors of 15 to 35% for short-duration storms, higher in some regions.
The difference between RCP 6.0 and RCP 8.5 can translate to a 10 to 15% increase in the design flood peak. On a flat floodplain, that can mean 200 to 400 mm of additional flood depth at a building platform.
Which RCP do councils require?
This is where practice varies significantly across New Zealand. Some councils have clear policy; others leave it to the engineer's professional judgement.
MfE (Ministry for the Environment) guidance recommends using RCP 8.5 for infrastructure with a design life beyond 2100, and at least RCP 6.0 for shorter-lived assets. In practice, most regional councils now expect RCP 8.5 at the 2081-2100 horizon for residential subdivision consent applications, on the basis that houses built today will still be occupied in 75 years.
On the Tukituki River flood study, RCP 8.5 at the 2081-2100 horizon was adopted for the primary flood hazard assessment. RCP 6.0 was also run for comparison, producing a Q100 approximately 12% lower.
How climate change uplift flows through the design
The uplift factor applies to the rainfall input, not directly to the flood flow output. The relationship between increased rainfall and increased flood peak is not one-to-one, because the catchment's losses (infiltration, initial abstraction) absorb some of the additional rainfall before it becomes runoff.
For a catchment with high CN (mostly impervious or saturated soils), a 20% increase in rainfall might produce a 22 to 25% increase in peak flow. For a more permeable catchment with lower CN, the same 20% rainfall increase might only produce a 15 to 18% increase in peak flow. This is why the SCS loss model and the climate change scenario need to be considered together, not independently.
Practical recommendations
For developers and landowners commissioning flood studies:
- Confirm your council's requirements early. Ring the consents team and ask which RCP scenario and time horizon they expect. This avoids rework.
- Expect RCP 8.5. Unless your council has a specific policy stating otherwise, assume RCP 8.5 at the 2081-2100 horizon will be required for residential development.
- Understand the cost implication. Higher climate change uplift means higher assessed flood levels, which may require raised building platforms, deeper foundations, or additional fill. The engineering cost difference between RCP 6.0 and RCP 8.5 is typically modest, but the earthworks cost can be significant.
- Ask for sensitivity analysis. A good flood study will report results for at least two RCP scenarios so that you can see the range of outcomes and make informed decisions about your development's risk tolerance.
Climate change uplift is now a standard requirement for NZ flood design. HIRDS provides the uplift factors, but the choice of RCP scenario (usually RCP 8.5 for residential) and time horizon directly affects flood levels and building platform requirements. Confirm your council's expectations before commissioning the study.